Section 4: Portable Retroreflectometers

Introduction

A critical issue associated with the retroreflectivity of
pavement markings is the ability to measure retroreflectivity.
Most pavement markings are manufactured on-site, and it is not practical
to remove a pavement marking so that the retroreflectivity can be
measured with a high level of accuracy. Instead, agencies and contractors
rely on portable retroreflectometers to measure the retroreflectivity
of pavement markings.

Portable pavement marking retroreflectometers have been available
for many years. Pavement markings are currently measured according
to 30-meter geometry, which represents retroreflectivity at a distance
typical of that which drivers view markings. Pavement markings
were previously measured at 15-meter geometry, and no simple conversion
exists between 15- and 30-meter measurements. Retroreflectometer
geometry is described in further detail in ASTM E 1710. Contact TxDOT’s
Materials Laboratory for a description of approved retroreflectivity
measurement devices. Note that different manufacturers’ retroreflectometers
will produce different retroreflectivity values for a given stripe,
and no accurate conversion factor exists.

Handheld vs. Mobile Units

Portable pavement marking retroreflectometers come in two
basic types: handheld and mobile. Examples of both types of retroreflectometers
are shown in Figure A-2.

Handheld retroreflectometers are much less expensive than
mobile units and are much easier to use and require very little
training. However, handheld units are inconvenient when a large
number of measurements are required or when measuring on roads with
high-traffic volumes. There are also safety issues related to the
use of handheld units as workers are often exposed to traffic while
measuring the retroreflectivity of a marking. Taking handheld measurements
often requires lane closures, increasing delay to motorists.

Mobile units on the other hand are much more expensive than
handheld units and require a significant amount of maintenance.
They also require a highly trained operator to take measurements.
However, mobile retroreflectometers produce a very large number
of measurements and allow for measurements to be taken at highway
speeds without exposure of personnel to traffic or lane closures.
Some state transportation agencies own mobile retroreflectometers,
although most agencies hire qualified contractors to perform mobile
retroreflectivity measurements if the measurements are taken to
determine contractor compliance with a performance or warranty specification.